High octane gasoline is required for modern gasoline engines. Formerly it was common to accomplish octane number improvement by the use of various lead-containing additives. As lead is phased out of gasoline for environmental reasons, it has become increasingly necessary to rearrange the structure of the hydrocarbons used in gasoline blending in order achieve higher octane ratings. Catalytic reforming and catalytic isomerization are two widely used processes for this upgrading.
The traditional gasoline blending pool normally includes C4 and heavier hydrocarbons having boiling points of less than 205° C. (395° F.) at atmospheric pressure. This range of hydrocarbon includes C4-C6 paraffins and especially the C5 and C6 normal paraffins which have relatively low octane numbers. The C4-C6 hydrocarbons have the greatest susceptibility to octane improvement by lead addition and were formerly upgraded in this manner. With eventual phase out of lead additives octane improvement was obtained by using isomerization to rearrange the structure of the paraffinic hydrocarbons into branched-chain paraffins or reforming to convert the C6 and heavier hydrocarbons to aromatic compounds. Normal C5 hydrocarbons are not readily converted into aromatics, therefore, the common practice has been to isomerize these lighter hydrocarbons into corresponding branched-chain isoparaffins. Although the C6 and heavier hydrocarbons can be upgraded into aromatics through hydrocyclization, the conversion of C6's to aromatics creates higher density species and increases gas yields with both effects leading to a reduction in liquid volume yields. Moreover, the health concerns related to benzene are likely to generate overall restrictions on benzene and possibly aromatics as well, which some view as precursors for benzene tail pipe emissions. Therefore, it is preferred to charge the C6 paraffins to an isomerization unit to obtain C6 isoparaffin hydrocarbons. Consequently, octane upgrading commonly uses isomerization to convert C6 and lighter boiling hydrocarbons.
Combination processes using isomerization and reforming to convert naphtha range feedstocks are well known. U.S. Pat. No. 4,457,832 uses reforming and isomerization in combination to upgrade a naphtha feedstock by first reforming the feedstock, separating a C5-C6 paraffin fraction from the reformate product, isomerizing the C5-C6 fraction to upgrade the octane number of these components and recovering a C5-C6 isomerate liquid which may be blended with the reformate product. U.S. Pat. No. 4,181,599 and U.S. Pat. No. 3,761,392 show a combination isomerization-reforming process where a full range naphtha boiling feedstock enters a first distillation zone which splits the feedstock into a lighter fraction which enters an isomerization zone and a heavier fraction that is charged as feed to a reforming zone. In both the '392 and '599 patents, reformate from one or more reforming zones undergoes additional separation and conversion, the separation including possible aromatics recovery, which results in additional C5-C6 hydrocarbons being charged to the isomerization zone.
The effluent from a reforming zone will contain a portion of hydrogen which may be used in the isomerization zone. Therefore combining the effluents to separate a stream containing hydrogen for recycle to the isomerization zone may be desirable, but a refiner may lose blending flexibility and may prefer to keep the isomerate and reformate separate from one another. Even while keeping the isomerate and reformate separate, cost savings may be achieved through integration of the net gas recovery zone between the reforming process and the isomerization process. The light ends from the isomerization zone effluent may be combined with the light ends of the reforming zone effluent and the combined light ends stream may be processed in a single net gas re-contacting zone. Portions of the resulting gas stream may be recycled.
The present invention involves a reforming zone and an isomerization zone where a portion of the isomerization zone light ends is directed to a net gas re-contacting zone in the reforming zone and where the isomerization zone uses a novel catalyst. Also, the reforming zone stabilizer overhead and the isomerization zone stabilizer overhead may use an integrated overhead receiver. The isomerization catalyst is a solid acid catalyst comprising a support comprising a sulfated oxide or hydroxide of at least an element of Group IVB (IUPAC 4) of the Periodic Table, a first component selected from the group consisting of at least one lanthanide-series element, mixtures thereof, and yttrium, and a second component selected from the group of platinum-group metals and mixtures thereof. In one embodiment of the invention, the atomic ratio of the first component to the second component is at least about 2. In another embodiment of the invention, the isomerization catalyst further comprises from about 2 to 50 mass-% of a refractory inorganic-oxide binder. In yet another embodiment of the invention, the isomerization catalyst further comprises from about 2 to 50 mass-% of a refractory inorganic-oxide binder having one or more platinum group metals dispersed thereon.